1. Field of the Invention
The present invention relates to hydraulic valves, and more particularly to spool type valves that have an integral filter for fluid flowing through the valve.
2. Description of the Related Art
A wide variety of machines have moveable members that are operated by an hydraulic actuator. For example, an internal combustion engine has a camshaft which is mechanically coupled to rotate with the crankshaft and which opens and closes cylinder intake and exhaust valves. Traditionally the camshaft timing was fixed at a setting that produced the best operation for all engine operating conditions. However, it has been recognized that engine performance can be improved if the valve timing varies as a function of engine speed, engine load, and other factors. Thus a hydraulic actuator is being used on some engines to vary the coupling relationship of the camshaft to the crankshaft. A solenoid operated valve control the application of pressurized fluid to operate the hydraulic actuator.
Over time, the hydraulic fluid flowing through a machine carries small particles, such as pieces of metal from the engine components. Those particles can block orifices in the valve or can become lodged so as to impede motion of valve components. The particles also may adversely affect operation of other elements of the hydraulic system. Some prior valves incorporated screens to prevent the small particles from entering the valve.